Thickness reduction control system



M. J. FETNER THICKNESS REDUCTION CONTROL SYSTEM Filed March 16, 1967 Feb. 18, 1969 I .l I I I I I I .I 02! Owl N0 a oomlqlmnflum m A n a 3:3 1 ||::||L I a: w I l r I l I [WWI/.1 I I l l l I I i l I l I I l I I I l l I I I Ill Qs IM\N-. MQ 1: .5926 J mm I 1: 2, Q q m muQZo a Q: a N2 55:8 B E 27 r 0 3 5 wa o 6L2 1024:5303 55:53 1 1055.53 55:09 o W l w m 3 3 L r ow: 8m .8 m, a? 8 Q- ,3 :9; NM .EwJ IQ r x "a 1 (Wm M o lg Gm 9 /n 2 m m QT hfiwwfio 5% mos: S a r V @0233 Om MARTIN J. FETNER his ATTORNEYS United States Patent Office THICKNESS REDUCTION CONTROL SYSTEM Martin J. Fetner, New London, Conn., assignor to General Dynamics Corporation, New York, N.Y., a corporation of Delaware Filed Mar. 16, 1967, Ser. No. 623,694

US. Cl. 72-9 15 Claims Int. Cl. BZlb 37/12, 37/02; G05d 13/62 ABSTRACT OF THE DISCLOSURE Reversible rolling mills in which the thickness reduction of rolled strip is controlled in one direction of pass by a mode of control based on the constant volume principle and in the other direction of pass by a follow-on mode of control.

This invention relates to installations which reduce the thickness of material. The invention is applicable to various sorts of such installations (as, say, bar mills, tube mills, and die drawing installations) which are employed to effect a reduction in thickness in one or more dimensions of materials of various compositions and shapes. For the purpose, however, of making clear the character of the invention, it is disclosed herein as being specifically embodied in control system for a reversible mill stand adapted to roll metal sheet or strip.

Such a stand has work rolls which reduce the thickness of the strip in accordance with the size of the gap or separation between those rolls. Because variations in thickness of the input strip to the rolls tends to a variable forcing apart of the roll sand because of other factors, it is necessary during rolling of the strip to control the roll separation in order to avoid undesirably large variations in thickness of the output strip. When the rolling is reversible, it is desirable that thickness reduction control be exercised in the course of passage of the strip through the stand in either rolling direction. Hence, it is preferable not only that the stand be reversible but also that the thickness control system for the stand be reversible in operation.

The commonest present type of mill control system relies only on follow-on control to correct variations in thickness of strip being rolled. In that follow-on system, an X-ray or other thickness gauge on the exit side of and disposed several feet from the stand is utilized to provide for a mill screwdown or like device a control signal derived from a sensing by the gauge of a deviation in the exit gage of the strip from a desired value. Because that sensed deviation is for a portion of the strip which is then several feet removed from the portion in the roll bite and subjected to the adjustment in roll separation effected by the control signal, the follow-on mode of control is alone unable to correct for variations in entry gage which are appreciable over a distance shorter than that separating the thickness gauge from the roll bite.

It has been proposed to overcome such shortcoming in follow-on systems by the use of lbefore-the-fact control systems wherein the roll separation is controlled in accordance with a measurement of strip entry gage which is made several feet before the strip enters the roll bite. That measurement is delayed in its effect on the roll separation for a time such that the adjustment of roll separation is coincident with the arrival at the roll bite of the portion of strip to which the measurement pertains. In addition, the signal which controls roll separation is derived not just from the delayed entry gage signal but also from signals of the entry and exit speeds of the strip. Those three signals are combined in a manner based on the constant volume principle so as to compensate for 3,427,836 Patented Feb. 18, 1969 factors (such as variation in the resilient stretching of the mill frame) which cannot be accurately corrected by the thickness gauge signal alone. Exemplary proposals of such before-the-fact constant volume mill control systems are to be found in US. Patent 3,015,974 (Orbom et a1.) and US. Patent 3,121,354 (Weremeychick et al.).

US. Patent 3,054,311 issued Sept. 18, 1962 to I. B. Murtland, Jr., proposes for a. reversible stand a reversibly operating constant volume control system having left and right-hand X-ray thickness gauges on opposite sides of the stand and left and right-hand tachometers on those opposite sides. A computer is fed by those gauges with signals of the actual entry and exit gages of the strip and of the entry and exit speeds of the strip. The computer derives from those input signals an output signal which controls a screw-down mechanism to adjust the roll spacing to the end of obtaining a desired exit gage for the strip. When the strip passes through the stand from left to right, the left and right thickness gauges feed the computer with signals of, respectively, the actual entry gage and the actual exit gage of the strip and, consonantly, the left and right tachometers feed the computer with signals of, respectively, the entry speed and exit speed of the strip. When the strip passes through the stand from right to left, the connections of the thickness gauges and of the tachometers to the computer are reversed so that the left and righthand thickness gauges feed the computer with signals of, respectively, the actual exit gage and the actual entry gage of the strip, and the left and right-hand tachometers feed the computer with signals of, respectively, the exit speed and the entry speed of the strip. That is, the Murtland, Jr. system is of such character that, for both directions of strip movement, both X-ray gauges are used, and the control signal for the screw-down mechanism is derived by the same computational mode of combining of the signals representing entry and exit gage and entry and exit speed.

The Murtland Jr. proposal is disadvantageous in that it requires iJWO thickness gauges as components of a control system for a reversible mill stand, and each such gauge may cost as much as $65,000. That control system is also disadvantageous in that, because the thickness reduction control signal is derived by a computational mode of signal combining for both directions of movement of the strip, the system is unduly complex and duplicative.

It is accordingly an object of the present invention to provide a thickness control system for a reversible mill stand or other reversible thickness-reducing installation wherein thickness gauge means is required on only one side of the installation, but thickness reduction control is obtained for both directions of movement of material through the installation.

Another object of the invention is to provide a control system of the sort described which is relatively simple and inexpensive in construction.

Still another object of the invention is to provide a method of correcting the thickness reduction effected on material being progressively reduced in thickness in successive steps by controlling such thickness reduction during alternate steps by, respectively, a before the fact mode of control and a follow-on mode of control.

These and other objects are realized according to the invention (as applied to a reversible thickness reducing installation) by providing a control system comprised of first and second alternatively selectable channels each adapted to provide a control signal for adjusting a wedge actuator or other actuator means adapted to correct the thickness reduction effected by the installation. Associated with and disposed on only one side of the installation is thickness gauge means selectively connectible to either one of those channels. Included in the first channel but excluded from the second channel is means to combine the thickness signal with other signals by a computational mode of signal-combining so as to provide from the first channel a control signal based on the constant volume principle.

During passage of material through the installation from the same side as that on which the thickness gauge means is disposed, such gauge means is connected to the first channel, and that channel provides for the actuator means the control signal derived as described by a computational mode of signal combining. Simultaneously, the second channel is disabled. Upon subsequent passage, however, of the material through the installation from the opposite side, the thickness gauge means is connected to the second channel which supplies the thickness signal alone to the actuator means to effect a follow-on [mode of control of the thickness reduction. During that subsequent passage, the first channel is disabled.

For a better understanding of the invention, reference is made to the following description of an exemplary embodiment thereof and to the accompanying drawing.

In the drawing, a strip of steel or other metal extends between a coil thereof on a left-hand reel 11 and a coil of the strip on a right-hand reel 12. Those reels are on opposite sides of an installation in the form of a mill stand 15 having lower and upper work rolls 16 and 17 disposed on opposite sides of strip 10 and providing working means for reducing the thickness of the strip. Work rolls 16 and 17 are backed by, respectively a lower back-up roll 18 and an upper back-up roll 19. Rolls 18 and 19 are mounted in respective roll chocks 20 and 21 disposed within a frame 22. At least the upper chock 21 is vertically slidable in guide ways formed for that chock on the inside of the frame.

During each passage of strip 10 through the stand 15, the thickness reduction effected on the strip by the rolls 16 and -17 is corrected by a device which may be a variable screw-down mechanism but which is, preferably, a wedge actuator of the type disclosed, for example, in the US. Patent No. 3,197,986 (Freedman et al.) or in copending application Ser. No. 405,749 filed Oct. 22, 1964, now Patent No. 3,355,925 and owned by the assignee hereof. As shown, device 30 bears against the top of roll chock 21 and is interposed between that chock and the upper crosshead of frame 22. In operation, actuator device 30 is adjusted in response to a control signal thereto to impart through elements 21 and 19 to work roll 17 a force which urges the work rolls toward each other so as to act in opposition to the reactive forces exerted on the work rolls by the strip 10 and tending to force those rolls apart. The force developed by device 30 varies in accordance with the control signal for that device to adjust the roll separation so as to smooth out variations in the entry thickness of strip 10 and to cause the exit thickness of that strip to conform to or approach a desired value.

Stand 15 is a reversible stand wherein the strip 10 is rolled to the desired final exit or output thickness or gage by passing the strip through the stand first in one direction and then in the other direction in a succession of passes which may exceed two in number. For the purposes of fully describing the shown system, it is necessary to consider only an initial left-to-right pass of the strip and a consecutively following right-to-left pass thereof.

To the end of making stand 15 reversible in operation, work rolls 16 and 17 are driven by a reversible motor 35, and reels 11 and 12 are coupled to selectively energizable drive motors 36 and 37, respectively. All three of motors -37 are controlled by a motor control unit 38 having a manually actuated selector switch handle 39 which may be thrown either to a left-right position or a right-left position. When handle 39 is thrown to its leftright position, that throwing establishes in unit 38 a set of internal connections (not shown) which (a) only partially energize motor 36 to cause it to act as a slight drag or brake on reel 11, (b) energize motor 35 to rotate the work rolls to drive strip 10 in the left-to-right direction through stand 15, and (c) fully energize motor 37 to cause reel 12 to act as a take-up reel and to develop tension in the rolled strip emerging from the stand. When handle 39 is thrown to its right-left position, unit 38 is actuated to (d) only partially energize motor 37 to cause it to act as a slight drag or brake on reel 12, (e) energize motor 35 to rotate the work rolls to drive the strip from right-to-left, and (f) fully energize motor 36 to cause reel 11 to take-up the rolled strip and to induce tension in the rolled strip then emerging from the stand 15. When handle 39 is at its central position, all of motors 35-37 are wholly deenergized.

The control signal for actuator device 30 is produced by a control system (to be described) and is arrived at during a left-to-right pass of the strip by an electrical computation based on the constant volume principle. The mathematics of such computation is as follows.

The derivation of the system equations starts with the basic premise that the volume of material entering the mill stand 15 equals the volume of material exiting the mill.

That is:

where V denotes volume and the subscripts 1 and 2 denote, respectively, an entry quantity and an exit quantity in connection both with the V terms and all subsequently used terms.

Then:

where:

L=length of material W=width of material G=gage of material For all practical purposes, the width of the material entering and exiting the mill does not vary significantly. Thus:

and Equation 2 reduces to:

Let G represent desired entry gage. Then from Equation 4 we have:

the desired entry gage, we can interpret this difference as an error signal. Thus:

for the strip 10. In practice, it is less convenient to measure the full entry gage than to measure the departure AG of actual full entry gage from a nominal entry gage value G which can be (but need not be) set equal to G such that:

When the right hand side of 10 is substituted in 9 for G there is obtained:

Expressions 11 and are the equations utilized in computing the control signal when, as stated, the strip is moving from left to right through stand 15. When the strip passes from right to left through the stand, the control signal is not computed in accordance with these equations but, instead, is in accordance with a follow-on mode of control later described in detail.

Turning now to the details of the control system, disposed on opposite sides of stand 15 are left and right speed measuring devices 45 and 46 having respective rolls 47 and 48 in contact with strip and coupled to, respectively, a left tachometer 50 and a right tachometer 51. Each of tachometers 50 and 51 responds to the driving by the strip of the roll coupled to that tachometer to produce an output in the form of a train of pulses characterized by a pulse repetition frequency proportional to the speed of the strip at that roll. During passage of the strip from left-to-right through stand 15, the rates at which pulses are produced by, respectively, tachometer '51 are measures of, respectively, the entry speed and the exit speed of strip 10 relative to that stand.

To the left of stand is disposed an X-ray thickness gauge comprised of an X-ray source 53 on one side of the strip, an X-ray detector head 54 on the other side of the strip and a signal generator section 55 coupled to head 54 to derive therefrom a signal (manifested on lead 56) of an instantaneous thickness value of the portion strip 10 then passing between the elements 54 and 55. To render that signal representative of the departure of the strip thickness from a selected nominal thickness value, signal generator section 55 is connected by lead 58 to a movable contact 59 in a first section of a switching means 60 in the form of a selector switch unit comprised of a number of switch sections. As indicated by dotted lines 61, the respective movable contacts of all those sections are ganged together and are selectively thrown to an upward position and to a downward position by manual throwing of a switch control handle 62 to, repectively, a leftward position (for left-to-right passage of strip 10) and a rightward position (for rightto-left passage of the strip).

When handle 62 is selectively thrown leftward and rightward, respectively, the movable contact 59 in the first section of switch unit 60 closes with, respectively, a fixed contact 63 and a fixed contact 64 of which both are parts of a gauge set-up sub-assembly 65. The closure of contacts 59 and 63 operates through lead 58 to render the thickness signal on lead 56 representative of a deviation A6 in strip thickness from a nominal entry thick ness value G which, as stated, may be set equal to G On the other hand, the closure of contacts 59 and 64 operates through lead 58 to render the thickness signal on lead 56 representative of a deviation AG in strip thickness from a nominal exit thickness value G which is ordinarily set equal to G the desired exit thickness for the strip on the second pass.

The selector switch 60 precedes the electronic circuits of the control system and controls the inputs to those circuits in the following manner. The thickness signal on lead 56 is connected through a switching section 67 and through a jumper lead 68 to a movable contact 70 disposed in another section of switch 60 and adapted to selectively close with, respectively, a fixed contact 71 and a fixed contact 72 when switch control handle 62 is thrown to be at, respectively, its leftward position and its rightward position. When contacts 70 and 71 are closed, the thickness signal AG is connected through fixed contact 71 to one input of a first electric signal channel 74 which develops a control signal for actuator 30 by a computational mode of signal combining based on the constant volume principle. When, contacts 70 and 72 are closed, then the thickness signal A6 is connected through the fixed contact 72 to a second channel 75 developing a control signal which adjusts actuator 30 by a follow-up mode of signal control. I

The pulse signals of strip entry speed are applied from tachometer 50 by lead 77 to a fixed contact 78 disposed in a section of switch unit 60 and engaged by a movable contact 79 when the control handle 62 of switch 60 is thrown leftward. Similarly, the pulse signals of strip exit speed are applied from tachometer 51 by lead 80 to a fixed contact 81 forming part of switch unit 60 and engaged by movable contact 82 when the handle 62 is thrown leftward. For that leftward position of 62, the entry and exit speed signals are supplied through, respectively, movable contact 79 and movable contact 82 to inputs for channel 74 which are separate from each other and from the thickness signal input. When handle 62 is thrown to its rightward position, the signals of strip entry speed and strip exit speed are disconnected from channel 74 and are not utilized in channel 75 or otherwise. Channels 74 and 75 are alternatively used to provide a control signal for actuator 30. The separate control signals from those two channels are adapted to be supplied to the actuator by separate leads 84 and 85 which, within element 30, are suitably isolated from each other so that either lead can be grounded without affecting the signal on the other lead. For grounding purposes, leads 84 and 85 are respectively connected (via leads 86 and 87) to fixed contacts 88 and 89 disposed in a section of switch unit 60 and adapted to be engaged, one or the other, by a grounded movable contact 90. When the control handle 62 of switch unit 60 is thrown leftward to enable channel 74 to supply a control signal to actuator 30, movable contact 90 closes with fixed contact 89 to ground lead 85 so as to disable channel 75 from supplying its control signal to the actuator. When handle 62 is thrown rightward to enable channel 75 to supply the control signal of that channel to actuator 30, movable contact 90 closes with fixed contact 88 to ground lead 84 to thereby disable the computing channel 74 from furnishing a control signal for the actuator.

Considering the circuitry and operation of channel 74, at the start of a desired left to right passage of the strip 10 through stand 15, the handle 39 of motor control unit 38 is thrown leftward to produce such movement, and the handle 62 of switch 60 is thrown leftward to enable channel 74 and disable channel 75. When so enabled, channel 74 receives entry speed pulses from tachometer 50, exit speed pulses from tachometer 51 and a thickness signal AG from X-ray gauge 53-55.

Within channel 74, the entry speed pulses are supplied to a pulse counter set by a manually controlled input 96 to a preset count of G so that the counter produces a clock pulse on output lead 97 whenever the received pulses accumulate to a count equal to G Concurrently with the production of that clock pulse, the counter 95 is cleared of the pulse count then accumulated therein. Hence, the counter is characterized by a succession of counting cycles of which each terminates by production of a clock pulse when the accumulated count of received entry speed pulses equals G Since those pulses have a repetition frequency proportional to the entry speed of strip 10- relative to stand 15, at any instant in a counting cycle the accumulated pulse count corresponds to the integral of such speed with respect to time over the time interval from the beginning of the cycle to that instant. That integral quantity corresponds, of course, to distance. Hence, the pulse count accumulated over that interval is a measure of the length L of strip 10 which has passed speed measuring device 45 in the course of such interval, and the production of the clock pulse occurs at the instant when L has attained the value of G The counter 95, therefore, implements the requirement of Expression 6 that the length L be set equal to G In the course of each counting cycle of counter 95, another counter 100 receives from tachometer 51 the pulses whose rate represents the exit speed of strip 10 from stand 15. At the end of each such cycle, the clock pulse from counter 95 is fed by leads 97 and 101 to the counter 100 to cause that counter to (l) read-out on connections 102 a digital signal of the exit speed pulse count then accumulated, and (2) clear itself of such then accumulated count so as to be ready to start a new counting cycle. The count accumulated per cycleby counter 100 is a measure of the length L of strip 10 which moves past speed measuring device 46 in the course of the interval required for the length L to equal G wherefore the signal on connections 102 is an L signa While connections 102 are represented in the drawing by a single lead, it is to be understood that such connections (and all other hereinafter described connections for a digital signal) are comprised of a plurality of leads of which each carries a l or binary signal corresponding to a respective one of the binary digits of a multidigit binary represented value.

The L signal is fed by connections 102 to a digital subtractor 105 into which a manually controlled input 106 introduces the digital quantity G from which L is to be subtracted. The clock pulse produced at the end of each counting cycle from counter 95 is fed by leads 97 and 107 to the subtractor 105 to cause that unit to perform the desired subtraction and to produce on a lead 108 a digital output per cycle corresponding to G1DL2.

Another occurrence during each counting cycle of counter 95 is that the AG thickness signal from X-ray gauge 53-55 is supplied via closed contacts 70, 71 to an analog-to-digital converter 110 which transforms that analog signal into digital form. The digital AG; signal is fed by lead 111 to a digital delay device 115 in the form of a multi-stage shift register.

Register 115 is controlled by pulses supplied by lead 116 from a pulse divider 120 driven by the entry speed pulses from tachometer 50 to provide one output pulse on that lead for each 11 input pulse to the divider where n is an integer greater than 1. The register responds to each such pulse on lead 116 to store in its first stage a sample of the digital signal on lead 111 and to shift forward by one stage each of the samples previously stored in the register. Thus, a given digital AG sample is shifted through register 115 at a rate proportional to the entry speed of strip 10. The value of n and the number of stages of the register are so selected that each stored AG sample arrives at the output stage of the register at about the same time as the arrival at the bite between workrolls 16 and 17 of the portion of strip from which that sample has been derived.

A read-out of the A6 digital sample then in the output stage of register 115 is produced at the end of each counting cycle of counter 95 by the clock pulse supplied from that counter via leads 97 and 121 to the register. The output AG digital sample is supplied by connections 122 to a digital subtractor unit 125 which also receives from connections 108 the digital sample of the quantity G L The clock pulse from counter 95 is fed by leads 97 and 126 to subtractor 125 to actuate that unit at the end of each counting cycle to subtract AG from so as to produce on connections 127 a digital sample of the remainder. That remainder corresponds to the quantity E in Expression 11. Accordingly, it will be evident that the components in channel 74 serve by a computational mode of signal combining to physically implement the mathematical relationships set out by Expression 11.

The E digital sample on connections 127 is fed to a digital accumulator 130 which stores a fixed substantial number of those samples and provides on connections 131 an output 2E representative of the average value of all the samples stored in the accumulator. At the end of a counting cycle, the clock pulse from counter 95 is fed by leads 97 and 132 to accumulator 130 to cause it to (1) accept in storage the E sample then on connections 127, and (2) make room for acceptance of that newest sample by clearing itself of the then oldest stored sample. The resulting 2E output is then held on connections 131 until the next clock pulse causes re-actuation of accumulator 130 and, perhaps, a subsequent change in value of the output 2E signal from the accumulator.

The held digital 2E signal is fed to a digital-to-analog converter 135 to be converted to an analog control signal supplied by lead 84 to actuator 30.

That control signal corrects the thickness reduction effected by rolls 16, 17 on strip 10 during the rightward passage of the strip in a manner which, briefly, is as follows:

X-ray gauge 53-55 senses a deviation AG in strip entry gage from the nominal value G Although such sensing is before-the-fact in that it takes place before the thickness deviation which is sensed is present at the roll bite, the adjustment of roll separation to correct for such deviation is made contemporaneous with the arrival of the deviation at the roll bite by delaying the AG signal in the manner disclosed. By combining the delayed AG signal with the L and G signals as described (and subject to the constraint that L =G and by deriving the control signal for actuating device 30 from such computational mode of signal combining based on the constant volume principle, the control signal is adapted to compensate for a number of factors (such as resilient stretching of the mill frame) which affect the accuracy with which the strip can be reduced to a desired thickness by the work rolls. Hence, during a rightward pass of strip 10, the described mill control system acts as a highly accurate closed loop system which responds to any sensed thickness deviation A6 to reduce to Zero or near zero both that deviation and the value of the control signal. As a result the strip which emerges from stand 15 during the rightward pass is a strip having a relatively flat exit gauge G which closely approaches the exit gauge G desired for that pass, and which varies only very slowly about that nominal thickness value G The once-rolled strip 10 is next rolled for a second time by passing it from right to left through stand 15. Such right to left passage of the strip is produced by throwing motor control handle 39 rightward and by likewise throwing switch control handle 62 rightward.

The rightward throwing of handle 62 effects the following changes in circuit connections. First, movable contacts 79 and 82 are opened with respect to fixed contacts 78 and 81 to disconnect from channel 74 the speed signals from tachometers 50 and 51. Second, movable contact 90 opens with fixed contact 89 to unground lead so as to enable channel 75 to supply a control signal to actuator 30. Third, movable contact closes with fixed contact 88 to ground lead 84 so as to disable channel 74 from supplying the control signal for actuator 30 (and so as, through lead 140, to discharge accumulator of the E samples stored therein). Fourth, movable contact 70 opens with fixed contact 71 and closes with fixed contact 72 so as to now connect the thickness signal from X-ray gauge 53-55 to the input of channel 75. Fifth, movable contact 59 opens with fixed contact 63 and then closes with fixed contact 74 to cause the gauge set-up subassembly 65 to modify through lead 58 the operation of the signal generator section 55 of the X-ray gauge in a manner whereby the thickness signal to channel 75 is a measure of the deviation A6 in actual exit gage of strip 10 from the exit gauge G desired for that strip on its second pass through the stand. That quantity G is, of course, smaller in value than the exit gage setting G employed during the first or left to right passage of the strip through the stand 15 As the strip 10 passes leftwardly through stand 15, it is further reduced in thickness by workrolls 16, 17 from its previously attained gage value G down to the smaller exit gage value of G In the course of such leftward rolling, the X-ray gauge 53-55 senses any deviation AG in strip exit gauge from the desired value 6 to produce a AG signal which is amplified in channel 75 by amplifier 150 and is then applied by lead 85 as a control signal to actuator 30. Whenever such a A6 signal is produced, the actuator is adjusted by it to correct the thickness re duction effected by rolls 16 and 17 so as to tend to reduce to zero both the deviation AG of the strip and the value of the AG signal.

Thus, as before, the thickness reduction of the strip is regulated in a closed loop manner. There is, however, a major difference between the thickness control provided during the first or rightward pass of strip through the stand and the thickness control provided during the subsequent leftward passage of the strip. Such difference is that, during the first pass, the mode of control is (as has been described) a computational mode of control based on the constant volume principle and providing a beforethe-fact correction in the thickness reducing action of the work rolls on the strip. During the subsequent leftward pass, however, the thickness gauge 53-55 senses a deviation AG in a portion of strip 10 only after that portion has passed through the work rolls and has moved a distance beyond. Hence, during such a leftward pass, the thickness reduction of the strip is controlled by the follow-on mode in the sense that a delay occurs between the time that a particular separation of the work rolls produces a deviation AG,,' in strip exit gage and the time that such deviation is first sensed and actuator 30 is adjusted to correct for that deviation.

Because of the delayed response which characterizes such follow-on mode of control, it might be thought that the described leftward rolling of strip 10 would have the disadvantage of prior art systems using follow-on thickness control of being unable (as such systems have been used) to produce flat exit gage for the strip emerging from the stand. The presently described system, does, however, produce fiat exit gage by the follow-on mode of control during the second pass of the strip for the reason that, during the first pass, the then-employed constant volume mode of control is effective to remove from the strip all thickness variations except those for which the change per unit length of strip is so small as to be in appreciable over the transport distance (during the second pass) between the bite of the rolls and the zone at which gauge 53-55 measures the deviation in strip thickness. Therefore, after the initial few feet of rolled strip has passed through stand 15, the deviation in gage measured at the X-ray gauge will ordinaril be identical to the deviation in gage at the roll bite, and the value of AG under those circumstances will fall to zero. If, however, a value AG does exist, it will be a small value and will change very slowly. Because of that slowness of change, the X-ray gauge (which may be locatedabout five feet beyond the end of the roll) will sense the deviation A6 quickly enough to compensate for A6 before it reaches a significant level.

The above-described embodiment of the invention being exemplary only, it is to be understood that additions thereto, modifications thereof and omissions therefrom can be made without departing from the spirit of the invention, and that the invention comprehends embodiments differing in form and/ or detail from that which has been specifically disclosed. For example, the thickness of the strip may be measured by means other than an X-ray gauge, and the logic system contained in channel 74 is only one of many possible analog, digital or analog-digital logic systems that may be used. While it is deemed preferable to first obtain a flat product by a rolling using a constant volume mode of thickness control and then obtain accurate gauge by a second rolling employing the follow-on mode of thickness control, those two rolling steps can, if desired, be reversed in order. The technique described herein of alternately using constant volume thickness control and follow-on thickness control during two rollings of the strip can, of course, be extended to more than two rollings and, when so extended, it becomes less important as to which mode of control is used first, although it is still deemed preferable to have the last rolling regulated by the follow-on mode. Finally, it is noted that the technique described herein of alternatel using a constant volume mode of thickness control and a followon mode of thickness control for successive thickness reductions of a strip is not only applicable where such successive reductions occur when the strip is successively passed in opposite directions through the same mill stand. Additionally, it is also applicable where the successive reductions of the material in question are obtained by passing such material in the same direction through successive stands of a multi-stand mill or other multi-stand thickness-reducing installation.

Accordingly, the invention is not to be considered as limited save as is consonant with the recitals of the following claims.

What is claimed is:

1. A control system for an installation wherein material is successively passed in opposite directions through working means to be reduced in thickness in a dimension transverse to the direction of passage, the thickness reduction being a function of the adjustment of means correcting the reduction effected by said working means, said system comprising, reversible drive means for said material, said drive means being selectively operable to pass said material through said working means in first one and then the other of said two opposite directions, thickness gauge means associated with that working means and disposed on only one side thereof to provide a signal of the thickness on that side of material passing through said working means, first and second electric signal channels electrically disposed between said gauge means and said correcting means, each of said channels being responsive to at least said thickness signal to each provide a respective electric output for adjustment of said correcting means as at least a function of that signal, said first and second channels being, respectively, inclusive and exclusive of computing means for combining said thickness signal with at least one other signal of a variable parameter of the material in passing through said working means so as to derive from said combined signals an electrical output in the form of a resultant signal for adjusting said correcting means, and channel switching means operable to select either one of said channels for effecting adjustment of said correcting means while disabling the nonselected channel from effecting such adjustment.

2. A system as in claim 1 in which said thickness signal is a signal of the actual deviation in the thickness of said material on said side of said working means from a selected nominal thickness value for such material on said side, said system further comprising means for switching the selected nominal thickness value between values which are representative of, respectively, nominal entry gauge and nominal exit gauge for material passing through said working means.

3. A system as in claim 1 in which said channel switching means is operable to select said first channel for effecting adjustment of said correcting means when said thickness signal is an entry gage signal for said material, and in which said computing means combines said thickness signal with signals representative of the entry and exit speeds relative to said working means of material passing through such means by a computational mode of signal combining based on the constant volume principle so as to derive said resultant signal.

4. A system as in claim 1 in which said working means comprises a rolling mill stand.

5. A system as in claim 4 in which said correcting means comprises means to vary the pressure exerted by the work rolls of said stand on said material.

6. A system as in claim 1 in which said thickness gauge means is an X-ray gauge.

7. A control system for an installation wherein material is successively passed in opposite directions through working means to be reduced in thickness in a dimension transverse to the direction of passage, the thickness reduction being a function of the adjustment of means correcting the reduction effected by said working means, said system comprising, reversible drive means for said material, said drive means being selectively operable to pass said material through said working means in first one and then the other of said two opposite directions, thickness gauge means associated with that working means and disposed on only one side thereof to provide a signal of the thickness on that side of material passing through said working means, and electric circuit means operable in response to at least said thickness signal to effect adjustment of said correcting means as a function of at least that signal when said material is passing through said working means in either one of said two directions.

8. A system according to claim 7 further comprising mode switching means for modifying the operation of said electric circuit means to effect adjustment of said correcting means in accordance with two different modes of signal control for effecting such adjustment.

9. A system according to claim 8 in which said two modes of signal control are, respectively, a computational mode of control based on the constant volume principle and a follow-on mode of exit gage signal control, said computational and follow-on modes of control being employed when said thickness signal is respectively, an entry gage signal for said material and an exit gage signal therefor.

10. A system as in claim 7 in which said thickness signal is a signal of the actual deviation in the thickness of said material on said side of said working means from a selected nominal thickness value for such material on said side, said system further comprising means for switching the selected nominal thickness value between values which are representative of, respectively, nominal entry gauge and nominal exit gauge for material passing through said working means.

11. A control system for a mill stand wherein material is successively passed in opposite directions through said stand between work rolls therein to be reduced in thickness, the thickness reduction being a function of the adjustment of means correcting said reduction by varying the pressure exerted by said rolls on said material, said system comprising, a thickness gauge disposed on one side of said stand to provide a signal of the thickness on that side of material passing through said stand, said thickness signal being representative of the actual deviation in thickness of said material from a selected nominal thickness value therefor, means for switching the selected nominal thickness value between values representative of, respectively, nominal entry gage and nominal exit gage for said material, first and second electric signal channels electrically interposed between said thickness gauge and said correcting means, each of said channels being responsive to an input of at least said thickness signal to each provide a respective electric output for adjustment of said correcting means, computing means included in said first channel to combine said thickness signal and signals of the entry and exit speeds relative to 6 said stand of said material so as to derive a first channel output 'for effecting adjustment of said correcting means in accordance with a mode of signal control based on the constant volume principle, signal transfer means included in said second channel to derive from said thickness signal a second channel output for effecting adjustment of said correcting means in accordance with a follow-on mode of exit gage signal control, and switch means to selectively apply said thickness signal as an input to said first channel and to said second channel and to effect adjustment of said correcting means by exclusively the channel to which such signal is applied, said thickness signal being applied to said first channel and to said second channel when said signal is representative of the deviation in actual thickness of said material from, respectively, nominal entry gage for said material and nominal exit gauge therefor.

12. The method of reducing the thickness of material by passing it through working means comprising, passing said material through said means in first one and then the second of two opposite directions, sensing the thickness of said material on only one side of said working means during passage of said material through said working means in both of said directions so as to derive from said sensing a thickness signal, and controlling by said signal the thickness reduction effected on said material by first and second modes of signal control when said material is passing through said working means in, respectively, said first direction and said second direction, said thickness signal being the only thickness signal utilized for controlling the thickness reduction effected on said material by said working means.

13. A method as in claim 12 in which said first and second modes of signal control are, respectively, a computational mode based on the constant volume principle and a follow-on mode of exit gage signal control.

14. A method as in claim 13 further comprising modifying said thickness signal to be representative of, respectively thickness deviation from nominal entry gage and thickness deviation from nominal exit gage during passage of said material through said working means in, respectively, said first direction and said second direction.

15. The method of reducing the thickness of material by passing it through working means comprising, progressively reducing such thickness by such means in consecutive thickness reducing steps, and correcting the thickness reduction effected in each of such steps by controlling such reduction during alternate ones of those steps by a computational mode of control based on the constaiit volume principle and by a follow-on mode of contro References Cited UNITED STATES PATENTS 3,015,974 1/1962 Orbom et al. 729 3,054,311 9/ 1962 Murtland 729 3,081,654 3/1963 Wallace 729 3,121,354 2/1964 Weremeychick et al 728 3,319,444 5/ 1967 Masterson 728 CHARLES W. LANHAM, Primary Examiner.

A. RUDERMAN, Assistant Examiner.

U.S. Cl. X.R. 7212, 16 

